Wave soldering (or flow soldering) is a widely used method of mass soldering electronic assemblies. It may be used, for example, for through-hole circuit boards, where the board is passed over a wave of molten solder, which laps against the bottom of the board to wet the metal surfaces to be joined. Another soldering technique involves printing of the solder paste on the soldering pads on the printed circuit boards followed by placement and sending the whole assembly through a reflow oven. During the reflow process, the solder melts and wets the soldering surfaces on the boards as well as the components. Another soldering process involves immersing printed wiring boards into molten solder in order to coat the copper terminations with a solderable protective layer. This process is known as hot-air leveling. A ball grid array joint or chip scale package is assembled typically with spheres of solder between two substrates. Arrays of these joints are used to mount chips on circuit boards.
As use of lead-free soldering materials becomes widespread, either due to environmental directives or pressure from the end users, so does the range of applications for such materials. High Ag solder alloys, such as SnAg3.0Cu0.5, have the benefits of excellent mechanical properties and good thermal reliability. However, the melting point of such alloys is about 217-221° C. This higher melting point requires reflow temperature of about 240-250° C., which may in certain cases be damaging to the printed circuit boards (PCBs) and electronic components.
There is a need for a lead-free solder alloy having a lower melting point than that of conventional high silver alloys, but with similar or more favourable mechanical and thermal properties.
The present invention aims to solve at least some of the problems associated with the prior art or to provide a commercially acceptable alternative.
Accordingly, in a first aspect, the present invention provides a lead-free, antimony-free solder alloy comprising:
(a) from 1 to 4 wt. % silver
(b) from 0.5 to 6 wt. % bismuth
(c) from 3.55% to 15 wt. % indium
(d) 3 wt. % or less of copper
(e) optionally one or more of the following elements                0 to 1 wt. % nickel        0 to 1 wt. % of titanium        0 to 1 wt. % manganese        0 to 1 wt. % of rare earths, such as cerium        0 to 1 wt. % of chromium        0 to 1 wt. % germanium        0 to 1 wt. % of gallium        0 to 1 wt. % of cobalt        0 to 1 wt. % of iron        0 to 1 wt. % of aluminum        0 to 1 wt. % of phosphorus        0 to 1 wt. % of gold        0 to 1 wt. % of tellurium        0 to 1 wt. % of selenium        0 to 1 wt. % of calcium        0 to 1 wt. % of vanadium        0 to 1 wt. % of molybdenum        0 to 1 wt. % of platinum        0 to 1 wt. % of magnesium        
(f) the balance tin, together with any unavoidable impurities.